THE INTERMOLECULAR VIBRATIONS OF AR-STYRENE AND AR-4-FLUOROSTYRENE COMPLEXES

One-color (1 + 1) resonance enhanced multiphoton ionization (REMPI) spectra are reported for styrene-Ar(n) clusters with n = 2, 3, and for 4-fluorostyrene-Ar(n) clusters with n = 2-5. These spectra are compared with previously recorded spectra of the mono-Ar complexes and discussed in relation to the structures of the clusters. For the mono-Ar complexes we have performed practically exact quantum calculations of the van der Waals vibrational frequencies and properties, starting from two different empirical atom-atom potentials. The intermolecular potentials are strongly anharmonic and, due to the low symmetry of these dimers, we find considerable mode mixing. As a consequence of the kinematic coupling between the Ar motion and the internal rotation, the bending frequencies depend considerably on the different rotational constants of the molecules. The order of the fundamental frequencies is the same for both dimers. For Ar-4-fluorostyrene the calculated vibrational frequencies agree well with the observed spectrum; the van der Waals side bands can thus be assigned in detail. For Ar-styrene the observed frequencies are less well reproduced, so we must conclude that the atom-atom potential used is substantially better for Ar-fluorostyrene than for Ar-styrene.